Diodes, especially TVS diodes, usually have a low trigger voltage and thus are suitable for on-chip ESD protection. Zener diodes are exemplary TVS diodes that are widely used for ESD protection. For example, an ESD protection device having a Zener diode coupled in parallel with two normal diodes that are coupled in series has been found suitable for protecting high-frequency CMOS I/O ports.
Zener diodes usually have a higher doping concentration than normal diodes. Due to this higher doping concentration, junction capacitance in a Zener diode is usually relatively high, which affects the speed of the circuit being protected. Moreover, at a high current, current crowding may occur due to high clamping voltage.